Summary

The distribution and quantitative recovery of isotopically labelled fertilizer N was studied in aPinus sylvestris L. stand in central Sweden. The investigation was carried out with root-isolated plots, area 31.2 m2, having a Scots pine aged 120–140 years in the centre of each plot. The fertilizer application rate was 100 kg ammonium nitrate-N per hectare, with either ammonium or nitrate being enriched with15N. The treatments comprised both a single and a split application.

The analyses of needle biomass, collected during the first growing season showed that the proportion of labelled N in Ntot was greater in current needles than in older needles. Moreover, the isotopic enrichment was higher in treatments with labelled nitrate than in those with equivalent amount of labelled ammonium.

Two growing seasons after fertilization the trees were felled and the distribution of labelled N in the system was determined. The study proved that the greatest proportion of labelled N in Ntot was in the needles and shoots of current and previous year's growth. The lowest proportion of labelled N was found in the stem wood and coarse roots.

The proportion of labelled N in needles and shoots showed a tendency to increase with height in the crown. As regards bark and wood, the corresponding figures increased from the base to the top of the stem, and was higher in bark than in wood.

Between 12 and 28% of the supplied N was recovered in trees, half of which in the needle biomass. In the above-ground parts of the shrub layer 3 to 13% of applied N was recovered. The corresponding recovery figure in the soil varied between 37 and 59% (excluding roots, but including litter and bottom layer). Between 5–8% was recovered in roots <30 mm. On the average, 79% of the supplied N was recovered in the studied part of the system. No significant differences in total15n recovery were found between treatments with ammonium or nitrate source of N, or those with different application techniques.

The labelled N not recovered in the investigated part of the system, had presumably been lost by leaching.

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